In cellular communication networks, radio basestations (RBS) provide radio network coverage over a coverage area or cell. In some implementations, radio basestations may be separated into one or more radio units and one or more baseband processing units, enabling, among other advantages, the optimising of radio unit placement. The radio units may be referred to as Remote Radio Units (RRUs) or as Radio Equipments (REs). The baseband processing units may be referred to as Digital Units (DUs), Baseband Units (BBUs) or as Radio Equipment Controllers (RECs). The Common Public Radio Interface (CPRI) specifies an internal interface protocol for RBS communication between REs and RECs.
Growing demand for cellular network data access has imposed increasing requirements upon the communication links between REs and RECs. For example, Long Term Evolution Time Division Duplexing (LTE-TDD) with 20 MHz bandwidth and eight receive antennas requires CPRI communication links between REs and RECs operating at 10 Gbit/s. Supporting such high speed signals with traditional electrical techniques is impractical, owing to realisation complexity, increased power consumption, cable costs and footprint. In addition, many RBS installations are implemented with a Main-Remote architecture, in which RE and REC are separated by a distance of up to a few kilometers. Optical fibres offer the possibility of transmitting high speed signals with very low loss, and consequently optical interconnection has proven the most viable technology for RBS communication between RE and REC.
Recent RBS development has focussed on the centralisation of baseband processing for RBSs. Centralised baseband processing offers efficiencies in the exploitation of computational resources as well as facilitating close coordination between cells covering the same or overlapping geographical areas. In a typical heterogeneous network deployment, a macro cell may be complemented with a set of coordinated small cells. Coordination among the set of small cells allows for spectrum sharing and reuse through the avoidance of interference, especially at cell borders.
In a centralised baseband processing deployment, multiple antenna sites host clusters of REs, which REs are connected with at least one pool of RECs, where baseband processing is performed. The communication links between REs and RECs in such deployments are collectively referred to as the fronthaul network. Optical networking based on Dense Wavelength Division Multiplexing (DWDM) is perceived as the ideal technology to serve the fronthaul network, owing to its capability to ensure high bandwidth, low latency, long reach, high fibre utilisation and high levels of resiliency.
In a typical Main-Remote architecture, each cluster of REs is connected to an REC in a one to one association via point to point connections. A significant increase of the processing optimisation could be achieved by moving to a more evolved networking scenario in which a pool of RECs could serve a large group of REs, which may thus be distributed over a wider geographical area. Such scenarios would allow for a greater optimisation of computational resources and for the inclusion a larger number of cells in a common area. In future evolutions of cellular network development, it is expected that baseband processing centralisation will be developed still further, with the concentration of RECs into a few specialised locations such as data centres. This development will require transport and switching solutions able to carry radio traffic across much larger distances.
Existing optical networking solutions for fronthaul networks offer transport over a distance of 10 to 15 km between REs and RECs. These existing solutions are based on Optical-Electro-Optical (OEO) conversion, with electrical switching at a hub node. Such solutions offer a high degree of scalability, owing to the reuse of wavelengths and to electrical switching, and a high degree of flexibility provided by sub-wavelength grooming. However the equipment in the hub node is not transparent to bit rates or to protocols. This lack of transparency means that future network evolution, which may be accompanied by the development of new transport and interface protocols, may require hardware replacement or reconfiguration in the fronthaul network. In addition, the extra power requirements for OEO conversion and CPRI switching increase the operating cost of such existing optical networking solutions.